During epithelial-mesenchymal interactions in numerous epidermal organ systems there is an assumed transfer of developmental information between tissues. This has been indirectly confirmed by heterotypic cell recombinant experiments, transfilter induction experiments and the diffusion of various isotopic precursors from one tissue to the other in vivo and in vitro. We described the fine structure of matrix vesicles found in the intracellular organic matrix interposed between tooth forming epithelia and mesenchyme. Some of these matrix vesicles contain RNAs which enhance cytodifferentiation in vitro. We isolated matrix vesicles during embryonic and early postnatal incisor tooth development. Are vesicles a morphological basis for the transfer of developmental information? What are the biological functions for intracellular vesicles? In which direction do they migrate? The employment of two strains of genetically defined mice (e.g., C57BL/1OSn and B10.D2/Sn differing only in one gene (H-2) system provides a means to selectively make alloantibody against histocompatibility alloantigens located specifically on the outer cell surfaces of all cells. Antibodies labeled with fluorescein, ferritin, or horseradish peroxidase, can be used to selectively mark either epithelial or mesenchymal membranes during experiments designed to mix the two tissues, each derived from a different strain of mice. Assuming that all matrix vesicles synthesized within either tissue type acquire an outer plasma membrane "marker" specific to the strain of mice, one can observe the actual transfer of material with immunofluorescent techniques. Subsequently, material actually transferred from tissue to tissue can be recovered from the recipient population of cells and from matrix vesicles which might, in addition, have a function in organic matrix formation, e.g., transport of enzymes, concentration of amorphous calcium and phosphates, structural glycoproteins, etc.